Semiconductor fabricators have used sol-gel techniques to produce dense thin films in semiconductors. The word sol-gel, however, does not describe a product but a reaction mechanism whereby a sol transforms into a gel. A sol is a colloidal suspension of solid particles in a liquid. One method of forming a sol is through hydrolysis and condensation reactions. These reactions cause a multifunctional monomer in a solution to polymerize into relatively large, highly branched particles. Many monomers suitable for polymerization are metal alkoxides. For example, a tetraethylorthosilicate (TEOS) monomer may be partially hydrolyzed in water by the reaction EQU Si(OEt).sub.4 +H.sub.2 O.fwdarw.HO--Si(OEt).sub.3 +EtOH
Reaction conditions may be controlled such that, on the average, each monomer undergoes a desired number of hydrolysis reactions to partially or fully hydrolyze the monomer. TEOS that has been fully hydrolyzed becomes Si(OH).sub.4. Once a molecule has been at least partially hydrolyzed, two molecules can then link together in a condensation reaction, such as EQU (OEt.sub.3 Si--OH+HO--Si(OH).sub.3 .fwdarw.(OEt).sub.3 Si--O--Si(OH).sub.3 +H.sub.2 O
or EQU (OEt).sub.3 Si--OEt+HO--Si(OEt).sub.3 .fwdarw.(OEt).sub.3 Si--O--Si(OEt).sub.3 +EtOH
to form an oligomer and liberate a molecule of water or ethanol. The Si--O--Si configuration in the oligomer formed by these reactions has three sites available at each end for further hydrolysis and condensation. Thus, additional monomers or oligomers can be added to this molecule in a somewhat random fashion to create a highly branched polymeric molecule from literally thousands of monomers.
Through continued reactions, one molecule in the sol may eventually reach macroscopic dimensions so that it forms a network that extends throughout the sol. At this point (called the gel point), the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, a gel can also be described as an open-pored solid structure enclosing a pore fluid. An oligomerized metal alkoxide, as defined herein, comprises molecules formed from at least two alkoxide monomers, but does not comprise a gel.